Li-Huang Lin

Evolution of spectrally discriminated spatial uniformity of line‐shaped plasmas

A space‐resolved soft x‐ray (SXR) transmission grating spectrometer and a SXR streak camera are used to investigate the evolution of spectrally discriminated spatial uniformity of line‐shaped plasmas produced by uniform laser illumination in line focus. It is found that the spatial nonuniformity of SXR emissions and ion turbulence occur during optical laser’s heating. Various instabilities can be excited due to the long scale length in line‐shaped plasmas to produce the phenomenon.

Space‐ and time‐resolved investigation of short wavelength x‐ray laser in Li‐like Ca ions

We have demonstrated the soft x‐ray amplification for lithium‐like Ca17+ 4f‐3d transition at 57.7 A with 900 ps, 1.05 μm drive laser pulse. The spatial distribution of the gain coefficient and temporal history of the lasing line emissions were also obtained.

Soft X-ray amplification by Li-like Al10+ and Si11+ ions in recombining plasmas

Experimental studies of soft X-ray lasers were carried out on the six-beam laser facility and the LF 12 laser facility of SIOM. Using a home-made one-dimensional spatially resolved grazing incidence grating spectrograph, XUV amplification has been observed in Li-like aluminum and silicon ions, by irradiation of slab targets with a line-focused laser. Based on time-integrated measurement, gain coefficients are 3.1 cm−1 for the 105.7 Å 5f−3d transition in Li-like Al ions, and 1.5 cm−1 and 1.4 cm−1 for the 88.9 Å 5f−3d and the 87.3 Å 5d−3p transitions in Li-like Si ions, respectively. The maximum gain × length products (GL) are about 2.5.

Soft x‐ray lasing and its spatial characteristics in a lithium‐like silicon plasma

Experimental study of soft x‐ray laser carried out on the LF12 Laser Facility of SIOM (Shanghai Institute of Optics and Fine Mechanics) is reported. Soft x‐ray amplifications were observed in lithium‐like silicon ions by line‐focused laser irradiation of slab targets. Based on the time‐integrated measurements, the gain coefficients are 1.5 and 1.4 cm−1 for the 5f–3d (88.9 A) and the 5d–3p (87.3 A) transitions of lithium‐like silicon ions, respectively. In addition, the spatial characteristics of the laser lines are also presented.

Filamentation and jets in line-focused laser-produced plasmas

Abstract In line-focused laser-produced plasma experiments on planar and cylindrical targets, we have observed the line-focused laser beam breakup into filaments as well as small-scale plasma jet-like structures. These phenomena might become obstacles in achieving optimum X-ray lasing in the type of experiments using line-focused laser-produced plasmas as a gain medium.

Large-scale jet structures in laser-produced plasmas

Abstract In line-focused laser-produced plasma experiments, we have observed large-scale plasma jets. These plasma jets are in appearance similar to “tips of ox horns”, and always appear at the boundaries between the hot plasmas and the cold target materials, or between two plasmas with different Z-numbers.

Extremely short pulse compression in bulk materials: a scheme for generating few cycle intense laser pulse

We investigate experimentally the self-compression behavior of high-power femtosecond pulses in normally dispersive solid bulk media with un-chirped laser pulses and negatively chirped laser pulses. It is demonstrated that high-power femtosecond laser pulses can be compressed by the nonlinear propagation in the transparent bulk media, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input laser intensity, with higher intensity corresponding to shorter compressed pulses. By the propagation in a piece of thin BK7 glass plate, a self-compression from 50fs to 20fs was achieved, with a compression factor of about 2.5. However, the output laser pulse was observed to be split into two peaks when the input laser intensity is high enough to generate supercontinuum and conical emission. When the input laser pulse is negatively chirped, the spectra of the pulse is reshaped and narrowed due to strong self-action effects, and the temporal pulse duration is found to be self-compressed, instead of broadening. With the increase in the input pulse intensity, the resulted self-compressed pulses became even shorter than the input laser pulse, and also shorter than sech2 transform-limited pulse according to the corresponding spectra. The self-compression scheme is simple and robust, and it is promising as a new pulse compression method to achieve intense laser pulses of few cycles.

Time and space resolved studies of recombination x-ray lasers

Abstract A novel experimental technique of time- and space-resolved X-ray diagnostics has been developed by coupling a stigmatic flatfield grating spectrometer to a soft X-ray streak camera to study the dynamics of recombination X-ray lasers. Time and space characteristics of Li-like Si X-ray lasers have been investigated and some interesting results have been obtained. Preliminary results with CaF2 target are given.

Intense laser pulse self-focusing and compression in normally dispersion bulk materials

The self-compression behavior of high power femtosecond pulses in normally dispersive solid bulk media was investigated with un-chirped laser pulses and negatively chirped laser pulses. It is demonstrated that high power femtosecond laser pulses can be compressed by the nonlinear propagation in the transparent bulk media, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input laser intensity, with higher intensity corresponding to shorter compressed pulses. By the propagation in a piece of thin BK7 glass plate, a self-compression from 50fs to 20fs was achieved, with a compression factor of about 2.5. When the input laser pulse is negatively chirped, the spectra of the pulse is reshaped and narrowed due to strong self-action effects, and the temporal pulse duration is found to be self-compressed, instead of broadening. In 2000, I. G. Koprinkov et al.[1] reported their observation of the ultrashort pulse self-compression (SC) in gases for the first time, which can be interpreted by the theory of gas-induced soliton [2]. Most recently, N. L. Wagner et al.[3] demonstrated a self-compression from 30fs to 13fs by propagating pulses inside a hollow waveguide filled with low-pressure argon gas. Rather than the effect of self-focusing (SF), they attributed their result mainly to the role of multiphoton ionization (MPI) excited by the high intensity of input pulses and the guiding in the hollow waveguide. Although the role of the group velocity dispersion (GVD) and the nonlinear parameters in solids differ by orders of magnitude from those in gases, it is considered that the temporal pulse shortening is also possible in bulk media as a result of self-channeling of high-peak-power femtosecond pulses [4], even to a few cycles [5-6]. However, these investigations are restricted to theoretical studies and primarily based on small input beam waist at the order of 10μm, long pulse duration (100fs level) and relatively low power, usually somewhat above the critical power for SF in solids at the order of MW. All of these parameters make the study somewhat far from the application of pulse compression of intense laser pulses. In this work, we use the laser pulses with a duration of ~50fs and peak power up to GW, and send them to a normally dispersive bulk medium (BK7 glass) in loose focusing condition, and SF is successfully demonstrated. On the other hand, although it is generally believed that SPM is associated with spectral broadening, it is not always the case if the input pulse is initially chirped. The sign of the initial chirp of the pulse decides whether the SPM will compress or broaden the spectrum. Spectral broadening occurs in the case that the initial pulse is positively chirped or no chirped. With negatively chirped pulses, SPM can result in spectral compression in normally dispersive nonlinear media. Spectral narrowing in optical fiber was theoretically studied with negatively chirped pulses [7]. We investigate the propagation of intense femtosecond pulses in the BK7 glass with initially negative chirp and we find that the spectrum of the output pulse is reshaped and narrowed due to strong self-action effects, and the temporal pulse duration is self-compressed, instead of broadening at the same time. With the increase in the input pulse intensity, the resulted self-compressed pulses became even shorter than the original laser pulse source, and also shorter than sech transform-limited pulse according to the corresponding spectrum. To our knowledge, this is the first report that the spectral narrowing and temporal compression can be obtained simultaneously in normally dispersive medium. The experimental setup is shown in Fig.1. The laser system used in our experiment is a commercially available chirped pulse amplification Ti:sapphire laser system (Spectral-Physics Spitfire) running at 1-KHz repetition rate, producing 0.5mJ/pulse, and about 50fs in duration with a central wavelength at 800nm. Typically, the beam quality parameter M is about 1.3 with 7mm in beam diameter (at 1/e2 of the peak intensity) and the bandwidth (FWHM) of the pulse is about 22nm. The laser beam first passes through an attenuator consisting of a half-wave plate (HWP) and a polarizer, which can continuously adjust the laser energy, QTuC4-3-INV

Recent progress in recombination X-ray laser research at Shanghai Institute of Optics and Fine Mechanics

In this paper, the results of the X-ray laser gain experiments of Li-like K and Ca ions, conducted recently at LF12 Laser Facility of SIOFM with KCl and CaF2 slab targets, will be presented. Also presented will be the space-resolved time history of ASE emission in the Li-like X-ray lasers and the in-situ calibration for the X-ray film used in the experiments.